The present invention relates generally to asphalt pavers of the floating screed type, and more particularly to the tow point connection used to join the floating screed and tractor portions of such a paver.
"Floating screed" asphalt finishing machines have provided an efficient and economical method of coating an old or new roadway with a compacted layer of asphalt aggregate for many years. Prior to the advent of such machines, the asphalt material was commonly raked by hand to grade and contour a roadway surface with unacceptable results.
Floating screed pavers are generally well known to those skilled in the art, as reflected by the disclosures contained in U.S. Pat. Nos. 3,997,277 issued to Swisher, Jr. et al.; 4,702,642 issued to Musil; 4,749,304 and 4,772,156 both issued to Craig; and 4,948,292 issued to Haven et al. Such a paver typically comprises a self-propelled paving vehicle (i.e., the tractor) having a hopper at its front end for receiving paving material, such as asphalt aggregate, from a dump truck.
A conveyor system on the tractor, in turn, transfers the paving material from the hopper rearwardly for discharge onto the underlying roadbed. Transversely arranged screw augers positioned at the rear end of the tractor assist in moving the paving material in a lateral direction with respect to the direction of movement of the paver, so that a relatively uniform volume of paving material is distributed across the portion of the roadbed in front of the floating screed.
This screed is commonly operated so as to "float" by virtue of being connected to the forwardly moving tractor by means of pivoted leveling arms. The screed, itself, performs two crucial functions. First, it physically levels any paving material lying higher than a predetermined height above the roadway surface, leaving a generally uniform thickness of such material. This function is enhanced by inclining the bottom surface of the screed so that its forward edge is higher than its rear edge, thereby providing a smaller area between the screed and the roadway, and a large dragging surface. The angle defined between the bottom surface of the screed and the roadway surface is called the "angle of attack."
The second function of the screed is to compact the dragged paving material in order to provide a uniform, smooth, durable pavement surface. In order to do this, the screed may actually vibrate against the pavement material.
The "tow point" is the point at which the leveling arms of the screed are attached to the paver tractor. In early pavers this point was a simple fixed pin connection. The thickness of the resulting paved mat, therefore, could only be controlled by means of altering the screed angle of attack.
Eventually, an additional feature was added to paver designs in order to allow the tow point to be moved vertically, causing a corresponding movement in the leveling arms and screed. This accommodated changes in the grade of the road surface by automatically fine tuning the initial setting of the screed angle of attack, thereby controlling the pavement mat thickness.
Two important aspects of paver operation are affected by the tow point connection between the tractor and screed units. First, it is generally preferred to add asphalt to the paver while the paver is still moving in order to avoid joints or dips and valleys in the resulting asphalt mat caused by stopping and starting movement of the machine. Thus, dump trucks full of the asphalt material are usually positioned ahead of, and in the path of, the paver. As the paver approaches the truck, or as the truck is backed towards the paver, push rollers on the front of the paver engage or are engaged by the dump truck's rearmost tires. At the moment of impact between the truck and paver, the paver is usually urged backwards, particularly if the truck has been backed into it. If the screed leveling arm is rigidly connected to the tractor, then the screed will likewise be moved backwards, which can damage the surface of the newly laid asphalt mat, causing bumps and valleys. Therefore, it would be advantageous to provide a paver which permits some degree of relative motion between the tractor and screed leveling arm connection so that the screed is not moved backwards when the paver and truck impact.
Second, the distance between the screed and the transverse auger of the tractor determines the size of aggregate particles which can pass between these two points. Therefore, the distance is usually adjusted to reflect the largest aggregate particle size being laid. In many asphalt mixes, especially those with high percentages of recycled asphalt, it is desirable to minimize the gap between the transverse auger and the front plate of the screed to minimize the buildup of hardened "dead" material, thereby reducing tearing of the asphalt mat and improving the quality of the end product. This gap is adjusted most easily at the tow point.